I think you can charge a lot more than 5c for this service. I think developers tend to think about margins as it relates to the service that the app is built on (this case twilio). Even though they charge 1c per text, you need to base your pricing on what a verified phone number is worth to someone using the service. If a verified number is worth 10$, you can probably charge more like 20-30% of that (maybe more).

I'm just regurgitating: http://www.kalzumeus.com/2011/12/19/productizing-twilio-appl...